A DEFINITION OF EVOLUTION 



mentally produced modifications are to have any evolutionary significance. 

 However, every serious experimental study designed to test this principle 

 has discredited it, with a single doubtful exception. This exception is pre- 

 sented by a series of experiments conducted by McDougall on learning in 

 rats. Rats were dropped into a tank of water from which there were two 

 exits, one lighted and one dark, but not always the same one. A rat leaving 

 by the lighted exit received an electric shock, while one leaving by the 

 dark exit received no shock. Thus the number of trials required for a par- 

 ticular rat to learn always to select the dark exit constituted a measure of 

 the speed of learning. These rats were then bred, and their descendants 

 were similarly studied. It appeared that the speed of learning increased 

 from generation to generation, and so McDougall concluded that learning, 

 an acquired trait par excellence, is inherited. Some serious criticisms have 

 been raised against McDougall's experiments. The genetic constitution 

 of his rats was not properly controlled, so that his initial breeding stock 

 may have been quite mixed as to intelligence levels. Neither the intensity 

 of the light nor of the electric shock was kept constant throughout the 

 experiment. Yet it is quite possible that variations in light intensity influ- 

 ence speed of learning in such an experiment, and McDougall himself 

 demonstrated that speed of learning varies directly with the intensity of 

 the shock. McDougall stated that adequate precautions were taken to 

 select breeding stock for subsequent generations at random. But he did 

 not describe his procedure, and it is entirely possible that the more intelli- 

 gent rats of each generation were selected to sire the next generation. In 

 this event, inheritance of acquired characteristics would not be necessary 

 in order to explain the improvement in learning ability from one genera- 

 tion to the next. Finally, during the course of the experiment, speed of 

 learning also increased among the control rats, that is among those bred 

 from untrained forebears. Thus it is probable that some unanalvzed 

 changes in the technique of the experiment may have been responsible 

 for all or part of the recorded increase in the speed of learning. But the 

 most serious defect is the fact that repetition of the experiment in other 

 laboratories has failed to produce similar results. In contrast to this pau- 

 city of positive evidence for the inheritance of acquired characteristics, 

 countless experiments have led only to negative results. For example, 

 Jewish boys have been circumcised for thousands of years, yet this has 

 not resulted in any tendency whatever toward reduction of the prepuce 

 among Jews. Examples could be multiplied indefinitely, but they all lead 

 to the same conclusion: acquired characters are not inherited. 



DARWIN 



So when J3arwin, jointly with Wallace, l)r()uglit forward the theory of 

 the origin of species by natural selection, there was no other evolutionary 

 theory to compete with it. The rapidity with which it achieved world-wide 

 acceptance by the majority of competent scientists is generally known, 

 as is also the bitter controversy which it produced among the lay public, 

 as well as among some scientists. It has been said that its rapid acceptance 



78 



